The digesters are either spherical, cylindrical, or egg-shaped, being constructed to revolve at a slow rate of speed, or fixed permanently in an upright position. Spherical boilers are usually 9 or 10 feet in diameter, the cylindrical digesters being 40 or 50 feet high and 12 or 15 feet diameter, the larger ones being capable of taking 20 tons of wood for each operation.
Fig. 33.—Wood Pulp Digester, partly in elevation, partly in section.
For the alkaline process the interior of the digester does not require any special treatment, but with the acid process the internal portion of the boiler is carefully lined with a thick layer of acid-resisting brick and cement.
The contents of the digester are heated by means of high-pressure steam, which is blown direct into the mass or passed through a coil lying at the bottom of the vessel. In the former case the steam is condensed by the liquor, the volume of which is consequently increased, while in the latter case the condensed steam is drawn off continuously from the pipes. Each system has its own particular advantages.
Different Kinds of Chemical Wood Pulp.—According to the method of treatment so the quality of the pulp varies. The chemicals used, the system of boiling, the temperature of digestion, the strength of the solutions, the duration of the cooking period, and, last but not least, the species of wood, are all determining factors in the value of the ultimate product.
Soda Pulp.—This is prepared by digesting wood with caustic soda in revolving boilers for eight or ten hours at a pressure of 60 to 80 lbs.
Sulphate Pulp.—Prepared by digesting the wood with a mixture of caustic soda, sulphide of soda, and sulphate of soda.
Sulphite Pulp.—The process most generally adopted for the manufacture of wood pulp is the treatment of the material in brick-lined digesters with bisulphite of lime for eight to nine hours at a pressure of 80 lbs.